The subject invention concerns a forming fabric for papermaking, cellulose and similar machines. The fabric comprises two layers of synthetic yarns, which layers are interconnected by synthetic yarns.
In the manufacture of paper on a papermaking machine, a slurry of fibres suspended in large quantities of water is allowed to flow onto a wire or cloth, which is permeable enough to permit drainage therethrough of the water in the slurry but prevents the fibres therein from passing through the cloth together with the water. Consequently, the fibres are retained and collected on the upper face of the cloth and are formed into a paper sheet. To avoid wire marking and fibre losses the paper-facing side of the fabric must be of a fine mesh weave. Originally single-layer fabrics, woven from metal threads were used. On account of their poor abrasion resistance metal wires have in later years been replaced by fabrics made from synthetic monofilament or multifilament yarns. In their single-layer version these fabrics are unsatisfactory inasmuch as they are a great deal more tensile and flexible than the metal wires. For this reason, single-layer synthetic fabrics have met with little success for use on wide and high-speed paper-making machines.
In an attempt to combine the fine-mesh yarn systems required for forming and sheet-formation with the degree of stability required by machine runability, various multi-layer constructions of forming fabrics have been developed. In the beginning was used a multi-layer forming fabric of a geometrical construction which in practice made it impossible to move the knuckles of the warp and weft yarns on the forming side of the fabric to a common outer plane. The result was considerable marking of the paper web. According to the teachings of SE Published Specification 72 11347 the forming side was improved by arranging each warp yarn in such a manner that in addition to interconnecting the two weft layers it also interlaces separately with the layer of weft yarns which in position of use of the fabric faces the material to be formed, in addition to which this layer of weft yarns as well as the warp yarns interconnecting these weft layers are essentially tangent to the (outer) plane of the fabric facing said material.
This improved forming face was retained in the construction disclosed in the Swedish Published Specification 74 12722-6. The characteristic features of the latter Patent Specification is that in the layer of weft yarns which in the position of use of the fabric faces the dewatering elements of the machine each warp yarns binds with every sixth weft yarn or less frequently.
The paper-facing side of the fabric must be of a fine mesh weave whereas the opposite side, which is exposed to considerable wear and abrasion, must be wear resistant and stable. Obviously, the requirements on the two sides of the fabric thus are in conflict with one another. One solution to these conflicting problems is found in the construction disclosed in Swedish Patent Specification 78 06764-2. The construction according to this publication consists of two complete weaves, each one comprising its separate, interconnected sets of warp yarns and weft yarns. The weave portion closest to the material to be formed as a rule consists of fine yarns which are woven into a fine-mesh weave construction whereas the bottom portion consists of coarser and more wear-resistant yarns which are woven into a more coarse mesh weave. The two weaves are interconnected either with the aid of separate binder yarns or by means of pairs of yarns which recur in sequence and which from the top cloth alternately interlace with the bottom cloth. The best fabric structure so far has proved to be one comprising a forming side weave of a two-shaft construction and a bottom side weave of either a two-shaft or a four-shaft construction, these weave constructions being interconnected by separate binder weft yarns. However, this construction is unsatisfactory because the binder weft yarns cause friction on the yarns of the two weaves, resulting in wear and rupture of the binder weft yarns.